1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus which can perform cone beam reconstruction, reconstruction processing apparatus, and image processing apparatus.
2. Description of the Related Art
There is an X-ray computed tomography apparatus (cone beam CT apparatus) which generates volume data by performing reconstruction of the projection data acquired by scanning a subject to be examined with cone beam X-rays. In cone beam CT, cone beam reconstruction such as FDK reconstruction (Feldkamp reconstruction) is useful.
Recently, with an increase in the number of element rows of an X-ray detector, the cone angle has increased. Owing to the influence of the cone angle, cone beam artifacts noticeably appear in the volume data generated on the basis of cone beam reconstruction.
CT scanning includes circular orbit scanning and helical scanning. The following problems arise in the respective scanning techniques.
(Circular Orbit Scanning)
Circular orbit scanning cannot acquire enough projection data required to obtain a complete solution. Line+Circle scanning is an application of circular orbit scanning. Line+Circle scanning can acquire enough projection data required to obtain a complete solution. However, Line+Circle scanning requires additional scanning. Additional scanning has the following problem. That is, to satisfy the reconstruction principle, the state of a subject in additional scanning needs to coincide with that in main scanning before additional scanning. More specifically, a change in contrast medium concentration and the body movement of the subject must not occur during main scanning and additional scanning. In practice, it is very difficult to satisfy this requirement. In addition, additional scanning complicates the workflow. Furthermore, additional scanning increases the dose to the subject.
(Helical Scanning)
Helical scanning can acquire enough projection data required to obtain a complete solution. Owing to the influence of the cone angle, however, noticeable cone beam artifacts appear in the volume data generated by directly performing cone beam reconstruction of the acquired projection data. Using the reconstruction method proposed by Alexander Katsevich in 2003 can obtain a complete solution on the basis of the projection data acquired by helical scanning. The reconstruction method by Katsevich is however limited in terms of helical pitch.